The invention concerns a heat transfer wall or interface between a high temperature heat source and an object to be heated such as the heater portion of a Stirling engine or a heat pipe operable with the Stirling heater. As is well-known, a temperature drop will occur across a heat transfer wall between a heat source and heat sink. Where the heat source involves combustion gases or other hot or potentially contaminating fluids from chemical or nuclear reactions, such heat source systems must be maintained separate from heater pipes that receive the heat from these fluids, and one known solution for this situation is a heat-transfer wall.
In seeking high thermal efficiency one design approach to this problem utilizes a single wall, i.e. common partition wall separating the heat source and heat sink systems, to minimize temperature drop and heat loss. The inherent problem in such design is that the two systems are not independent and separable from each other, because they are either permanently combined into one complex system due to their common wall, or there must be a complete dismantling of one system in order to get to the other system. Another design approach would be to have two independent chambers or containers, with one wall of each chamber placed close together or in contact, thus requiring heat transfer across both walls. The obvious disadvantages here are numerous: first there is temperature drop and heat loss through two walls as opposed to one; second there is further temperature drop through the gap between the two walls if such gap occurs; and third, if the two walls are in contact in a high temperature environment, over a period of time the walls may become joined together by various chemical and/or metalurgical reactions, rendering it difficult to separate them during dismantling.
In the particular situation of a Stirling engine which requires a heat input at temperature in the range of 1500.degree. F., to its sodium heat pipe from a heat source such as chemical combustion using lithium sulfide (LiSF.sub.6) or a nuclear reactor, it is critical that the chemical substances be securely maintained within the heat source chamber. If a wall of this chamber is a common wall for the engine heater, and heat source, the assembly of either will be greatly complicated. The intermediate source could well be a sodium heat pipe separated from the chemical reactor by a single metal wall also with the same problems mentioned above.
Thus, the problem faced was to provide an interface between a heat source and the engine heater which would have the highest possible efficiency and heat transfer properties, and thus the lowest temperature drop across the interface, but which would also permit the two components to be easily and fully separated from each other while leaving each intact. The use of two such separate components with their own walls placed closely adjacent led to the problems discussed above namely, the walls becoming bound to each other during the long heat transfer, and heat losses through the two walls and the gap between them, and finally if the walls were placed in contact, different rates of expansion would cause various undesirable stress situations of one wall against the other.
This invention concerns a heat transfer wall or interface between a high temperature heat source and an object to be heated such as the heater portion or heat pipe of a Stirling engine. For such engines common heat sources include combustion gases, or other hot or potentially contaminating fluids from chemical or nuclear reactions. In these situations a heat transfer wall or interface is provided to maintain the heat source system separated from heater pipes of the engine while allowing heat transfer through such wall. The rate of heat flow through and the temperature drop across such walls are parameters of concern in these composite systems. In the case of a Stirling engine, heat transferred through the interface wall is absorbed in fluid medium of a heat pipe and transported by this medium to heater pipes of the engine. Where the interface comprises a single wall of good heat conductivity, the temperature drop, .DELTA. T, and rate of heat transfer will be optimized; however the severe disadvantage is that the interface is a common wall of two closed chambers. Separation of one chamber with the wall from the other, leaves the other chamber open and unsealed. In effect the two systems are not separable without dismantling at least one, and breaking the gas seal.
A contrary approach requires two independent chambers, having closely adjacent or touching walls. Here the heat transfer efficiency is greatly reduced by the cumulative temperature drops across both walls plus across the gap between walls. Use of either of the above structural arrangements substantially compromises benefits of the other. The new invention does in fact combine structural advantages of two separate chambers while approaching the thermodynamic advantages of a single wall.